A Holter monitor report from an overseas customer was recently returned with a note pointing out that we had missed the diagnosis of intermittent pre-excitation - the Wolff-Parkinson-White Syndrome.

On inspection of the tracings, sure enough there were frequent complexes that had a short PR interval and a delta wave (highlighted).Did we actually miss this diagnosis?On careful inspection, each of these complexes occurs when the rhythm transitions from a narrow sinus rhythm with a normal P wave and PR interval to a broad QRS rhythm with no P wave. This is an idioventricular rhythm, rate 60-70 bpm and the complexes with a short PR interval and delta wave are fusion beats.Remember fusion causes confusion. This can be called pseudo-WPW.With Wolff-Parkinson-White syndrome, pre-excitation can be intermittent

And alternating!

Sometimes the delta wave is unusual.

What other situations can cause pseudo-WPW?

Late ventricular ectopics (VE) and fusion (F1, F2), all highlighted.

This is an example of isorhythmic AV dissociation with the alternating junctional escape beats fused with the sinus P waves resulting in a pseudo-WPW appearance (highlighted).